If you got a free certificate using letencrypt-nosudo and you need to renew your certificate, you probably checked github and the readme to know which steps you need to go through.

It’s not said explicitly but updating your certificate is the same way than asking a new one. You just have to skip the steps where you create your private key and domain’s csr.

So, in another words, you just have to run the following command in the folder where you have the letencrypt-nosudo client :
$> python sign_csr.py –public-key your_user.pub your_domain.csr > your_updated_signed.crt
Replace your_user.pub and your_domain.csr by what you already have. Also, if you are on manjaro (or archlinux) like me, use python2 instead of python.